1. Field of Invention
This invention relates to an optical recording disc and a method for recording and reproducing such an optical recording disc.
2. Discussion of the Background
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. For example, Specification (Ver. 1.0) for DVD-RAM which is a phase change optical recording disc was published in July, 1997. A phase change optical recording medium is a medium wherein a laser beam is directed to the recording layer to change its crystalline state whereupon a change of reflectance by the crystallographic change is detected for reproduction of the information. The phase change optical recording media are of great interest since the optical system of the drive unit used for their operation is simple as compared with magneto-optical recording media.
When information is recorded in the optical recording medium of phase change type, the laser beam applied is of high power (recording power) that the recording layer is heated to a temperature higher than the melting point. In the region where the recording power is applied, the recording layer is melted and thereafter quenched to form an amorphous record mark. When the record mark is erased, a laser beam of relatively low power (erasing power) is applied so that the recording layer is heated to a temperature higher than the crystallizing temperature and lower than the melting temperature. The record mark to which the laser beam of erasing power is applied is heated to a temperature higher than the crystallizing temperature and then allowed to slowly cool to recover the crystalline state. Accordingly, in the optical recording media of the phase change type, the medium can be overwritten by modulating the intensity of a single laser beam.
Phase change-type optical recording discs which found relatively widespread use include PD and CD-RW. These optical recording discs are of the format wherein a spiral guide groove (which is hereinafter referred to as a groove) formed on the surface of the substrate is used for the recording track. In such disc, the recording track is continuous, and jump of optical pickup is not required even when the information is continuously recorded and reproduced in two or more turns of the recording tracks.
Recently, so-called land/groove recording wherein land (the region between two adjacent grooves) in addition to the groove is used for the recording track in order to increase the recording density has gone into practical use. When a disc having a single spiral groove is recorded by such land/groove recording, the recording track will be discontinuous, and the optical pickup is required to jump to the next track, namely, the adjacent land or groove once per one rotation of the disk. Such jumping to the adjacent recording track is herein referred to as a track jump.
Therefore, it is general in the optical recording disc for land/groove recording to preform a synchronization mark pattern (typically a pit train) in the land or groove of the substrate so that the optical pickup can read such pattern to effect the track jump. Such synchronization mark pattern may be formed in the disc substrate simultaneous with the injection molding of the disc, and for such simultaneous formation of the pattern, a pit train-forming pattern may formed in the stamper which functions as the mold in the injection molding. Production of such stamper adapted for simultaneous formation of the pit train in the groove or on the land, however, is quite difficult since track pitch has been reduced with the increase in the recording density, and formation of an accurate pattern is difficult. Such difficulty has been reflected on the track jump error.